NAME

perlhack - How to hack at the Perl internals

DESCRIPTION

This document attempts to explain how Perl development takes
place, and ends with some suggestions for people wanting to
become bona fide porters.
The perl5-porters mailing list is where the Perl standard
distribution is maintained and developed. The list can get
anywhere from 10 to 150 messages a day, depending on the
heatedness of the debate. Most days there are two or three
patches, extensions, features, or bugs being discussed at a
time.
A searchable archive of the list is at either:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
or
http://archive.develooper.com/perl5-porters@perl.org/
List subscribers (the porters themselves) come in several
flavours. Some are quiet curious lurkers, who rarely pitch
in and instead watch the ongoing development to ensure
they're forewarned of new changes or features in Perl. Some
are representatives of vendors, who are there to make sure
that Perl continues to compile and work on their platforms.
Some patch any reported bug that they know how to fix, some
are actively patching their pet area (threads, Win32, the
regexp engine), while others seem to do nothing but com-
plain. In other words, it's your usual mix of technical
people.
Over this group of porters presides Larry Wall. He has the
final word in what does and does not change in the Perl
language. Various releases of Perl are shepherded by a
"pumpking", a porter responsible for gathering patches,
deciding on a patch-by-patch, feature-by-feature basis what
will and will not go into the release. For instance,
Gurusamy Sarathy was the pumpking for the 5.6 release of
Perl, and Jarkko Hietaniemi was the pumpking for the 5.8
release, and Rafael Garcia-Suarez holds the pumpking crown
for the 5.10 release.
In addition, various people are pumpkings for different
things. For instance, Andy Dougherty and Jarkko Hietaniemi
did a grand job as the Configure pumpkin up till the 5.8
release. For the 5.10 release H.Merijn Brand took over.
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Larry sees Perl development along the lines of the US
government: there's the Legislature (the porters), the Exe-
cutive branch (the pumpkings), and the Supreme Court
(Larry). The legislature can discuss and submit patches to
the executive branch all they like, but the executive branch
is free to veto them. Rarely, the Supreme Court will side
with the executive branch over the legislature, or the leg-
islature over the executive branch. Mostly, however, the
legislature and the executive branch are supposed to get
along and work out their differences without impeachment or
court cases.
You might sometimes see reference to Rule 1 and Rule 2.
Larry's power as Supreme Court is expressed in The Rules:
1 Larry is always by definition right about how Perl
should behave. This means he has final veto power on the
core functionality.
2 Larry is allowed to change his mind about any matter at
a later date, regardless of whether he previously
invoked Rule 1.
Got that? Larry is always right, even when he was wrong.
It's rare to see either Rule exercised, but they are often
alluded to.
New features and extensions to the language are contentious,
because the criteria used by the pumpkings, Larry, and other
porters to decide which features should be implemented and
incorporated are not codified in a few small design goals as
with some other languages. Instead, the heuristics are
flexible and often difficult to fathom. Here is one
person's list, roughly in decreasing order of importance, of
heuristics that new features have to be weighed against:
Does concept match the general goals of Perl?
These haven't been written anywhere in stone, but one
approximation is:
1. Keep it fast, simple, and useful.
2. Keep features/concepts as orthogonal as possible.
3. No arbitrary limits (platforms, data sizes, cultures).
4. Keep it open and exciting to use/patch/advocate Perl everywhere.
5. Either assimilate new technologies, or build bridges to them.
Where is the implementation?
All the talk in the world is useless without an imple-
mentation. In almost every case, the person or people
who argue for a new feature will be expected to be the
ones who implement it. Porters capable of coding new
features have their own agendas, and are not available
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to implement your (possibly good) idea.
Backwards compatibility
It's a cardinal sin to break existing Perl programs.
New warnings are contentious--some say that a program
that emits warnings is not broken, while others say it
is. Adding keywords has the potential to break pro-
grams, changing the meaning of existing token sequences
or functions might break programs.
Could it be a module instead?
Perl 5 has extension mechanisms, modules and XS, specif-
ically to avoid the need to keep changing the Perl
interpreter. You can write modules that export func-
tions, you can give those functions prototypes so they
can be called like built-in functions, you can even
write XS code to mess with the runtime data structures
of the Perl interpreter if you want to implement really
complicated things. If it can be done in a module
instead of in the core, it's highly unlikely to be
added.
Is the feature generic enough?
Is this something that only the submitter wants added to
the language, or would it be broadly useful? Sometimes,
instead of adding a feature with a tight focus, the
porters might decide to wait until someone implements
the more generalized feature. For instance, instead of
implementing a "delayed evaluation" feature, the porters
are waiting for a macro system that would permit delayed
evaluation and much more.
Does it potentially introduce new bugs?
Radical rewrites of large chunks of the Perl interpreter
have the potential to introduce new bugs. The smaller
and more localized the change, the better.
Does it preclude other desirable features?
A patch is likely to be rejected if it closes off future
avenues of development. For instance, a patch that
placed a true and final interpretation on prototypes is
likely to be rejected because there are still options
for the future of prototypes that haven't been
addressed.
Is the implementation robust?
Good patches (tight code, complete, correct) stand more
chance of going in. Sloppy or incorrect patches might
be placed on the back burner until the pumpking has time
to fix, or might be discarded altogether without further
notice.
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Is the implementation generic enough to be portable?
The worst patches make use of a system-specific
features. It's highly unlikely that nonportable addi-
tions to the Perl language will be accepted.
Is the implementation tested?
Patches which change behaviour (fixing bugs or introduc-
ing new features) must include regression tests to ver-
ify that everything works as expected. Without tests
provided by the original author, how can anyone else
changing perl in the future be sure that they haven't
unwittingly broken the behaviour the patch implements?
And without tests, how can the patch's author be confi-
dent that his/her hard work put into the patch won't be
accidentally thrown away by someone in the future?
Is there enough documentation?
Patches without documentation are probably ill-thought
out or incomplete. Nothing can be added without docu-
mentation, so submitting a patch for the appropriate
manpages as well as the source code is always a good
idea.
Is there another way to do it?
Larry said "Although the Perl Slogan is There's MoreThan One Way to Do It, I hesitate to make 10 ways to do
something". This is a tricky heuristic to navigate,
though--one man's essential addition is another man's
pointless cruft.
Does it create too much work?
Work for the pumpking, work for Perl programmers, work
for module authors, ... Perl is supposed to be easy.
Patches speak louder than words
Working code is always preferred to pie-in-the-sky
ideas. A patch to add a feature stands a much higher
chance of making it to the language than does a random
feature request, no matter how fervently argued the
request might be. This ties into "Will it be useful?",
as the fact that someone took the time to make the patch
demonstrates a strong desire for the feature.
If you're on the list, you might hear the word "core" ban-
died around. It refers to the standard distribution.
"Hacking on the core" means you're changing the C source
code to the Perl interpreter. "A core module" is one that
ships with Perl.
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Keeping in sync
The source code to the Perl interpreter, in its different
versions, is kept in a repository managed by a revision con-
trol system ( which is currently the Perforce program, see
http://perforce.com/ ). The pumpkings and a few others have
access to the repository to check in changes. Periodically
the pumpking for the development version of Perl will
release a new version, so the rest of the porters can see
what's changed. The current state of the main trunk of
repository, and patches that describe the individual changes
that have happened since the last public release are avail-
able at this location:
http://public.activestate.com/pub/apc/
ftp://public.activestate.com/pub/apc/
If you're looking for a particular change, or a change that
affected a particular set of files, you may find the PerlRepository Browser useful:
http://public.activestate.com/cgi-bin/perlbrowse
You may also want to subscribe to the perl5-changes mailing
list to receive a copy of each patch that gets submitted to
the maintenance and development "branches" of the perl repo-
sitory. See http://lists.perl.org/ for subscription infor-
mation.
If you are a member of the perl5-porters mailing list, it is
a good thing to keep in touch with the most recent changes.
If not only to verify if what you would have posted as a bug
report isn't already solved in the most recent available
perl development branch, also known as perl-current, blead-
ing edge perl, bleedperl or bleadperl.
Needless to say, the source code in perl-current is usually
in a perpetual state of evolution. You should expect it to
be very buggy. Do not use it for any purpose other than
testing and development.
Keeping in sync with the most recent branch can be done in
several ways, but the most convenient and reliable way is
using rsync, available at ftp://rsync.samba.org/pub/rsync/ .
(You can also get the most recent branch by FTP.)
If you choose to keep in sync using rsync, there are two
approaches to doing so:
rsync'ing the source tree
Presuming you are in the directory where your perl
source resides and you have rsync installed and
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available, you can "upgrade" to the bleadperl using:
# rsync -avz rsync://public.activestate.com/perl-current/ .
This takes care of updating every single item in the
source tree to the latest applied patch level, creating
files that are new (to your distribution) and setting
date/time stamps of existing files to reflect the blead-
perl status.
Note that this will not delete any files that were in
'.' before the rsync. Once you are sure that the rsync
is running correctly, run it with the --delete and the
--dry-run options like this:
# rsync -avz --delete --dry-run rsync://public.activestate.com/perl-current/ .
This will simulate an rsync run that also deletes files
not present in the bleadperl master copy. Observe the
results from this run closely. If you are sure that the
actual run would delete no files precious to you, you
could remove the '--dry-run' option.
You can than check what patch was the latest that was
applied by looking in the file .patch, which will show
the number of the latest patch.
If you have more than one machine to keep in sync, and
not all of them have access to the WAN (so you are not
able to rsync all the source trees to the real source),
there are some ways to get around this problem.
Using rsync over the LAN
Set up a local rsync server which makes the rsynced
source tree available to the LAN and sync the other
machines against this directory.
From http://rsync.samba.org/README.html :
"Rsync uses rsh or ssh for communication. It does not need to be
setuid and requires no special privileges for installation. It
does not require an inetd entry or a daemon. You must, however,
have a working rsh or ssh system. Using ssh is recommended for
its security features."
Using pushing over the NFS
Having the other systems mounted over the NFS, you
can take an active pushing approach by checking the
just updated tree against the other not-yet synced
trees. An example would be
#!/usr/bin/perl -w
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use strict;
use File::Copy;
my %MF = map {
m/(\S+)/;
$1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime
} `cat MANIFEST`;
my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2);
foreach my $host (keys %remote) {
unless (-d $remote{$host}) {
print STDERR "Cannot Xsync for host $host\n";
next;
}
foreach my $file (keys %MF) {
my $rfile = "$remote{$host}/$file";
my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9];
defined $size or ($mode, $size, $mtime) = (0, 0, 0);
$size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next;
printf "%4s %-34s %8d %9d %8d %9d\n",
$host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime;
unlink $rfile;
copy ($file, $rfile);
utime time, $MF{$file}[2], $rfile;
chmod $MF{$file}[0], $rfile;
}
}
though this is not perfect. It could be improved
with checking file checksums before updating. Not
all NFS systems support reliable utime support (when
used over the NFS).
rsync'ing the patches
The source tree is maintained by the pumpking who
applies patches to the files in the tree. These patches
are either created by the pumpking himself using "diff
-c" after updating the file manually or by applying
patches sent in by posters on the perl5-porters list.
These patches are also saved and rsync'able, so you can
apply them yourself to the source files.
Presuming you are in a directory where your patches
reside, you can get them in sync with
# rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
This makes sure the latest available patch is downloaded
to your patch directory.
It's then up to you to apply these patches, using
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something like
# last=`ls -t *.gz | sed q`
# rsync -avz rsync://public.activestate.com/perl-current-diffs/ .
# find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch
# cd ../perl-current
# patch -p1 -N <../perl-current-diffs/blead.patch
or, since this is only a hint towards how it works, use
CPAN-patchaperl from Andreas Kvnig to have better con-
trol over the patching process.
Why rsync the source tree
It's easier to rsync the source tree
Since you don't have to apply the patches yourself, you
are sure all files in the source tree are in the right
state.
It's more reliable
While both the rsync-able source and patch areas are
automatically updated every few minutes, keep in mind
that applying patches may sometimes mean careful
hand-holding, especially if your version of the "patch"
program does not understand how to deal with new files,
files with 8-bit characters, or files without trailing
newlines.
Why rsync the patches
It's easier to rsync the patches
If you have more than one machine that you want to keep
in track with bleadperl, it's easier to rsync the
patches only once and then apply them to all the source
trees on the different machines.
In case you try to keep in pace on 5 different machines,
for which only one of them has access to the WAN,
rsync'ing all the source trees should than be done 5
times over the NFS. Having rsync'ed the patches only
once, I can apply them to all the source trees automati-
cally. Need you say more ;-)
It's a good reference
If you do not only like to have the most recent develop-
ment branch, but also like to fix bugs, or extend
features, you want to dive into the sources. If you are
a seasoned perl core diver, you don't need no manuals,
tips, roadmaps, perlguts.pod or other aids to find your
way around. But if you are a starter, the patches may
help you in finding where you should start and how to
change the bits that bug you.
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The file Changes is updated on occasions the pumpking
sees as his own little sync points. On those occasions,
he releases a tar-ball of the current source tree (i.e.
perl@7582.tar.gz), which will be an excellent point to
start with when choosing to use the 'rsync the patches'
scheme. Starting with perl@7582, which means a set of
source files on which the latest applied patch is number
7582, you apply all succeeding patches available from
then on (7583, 7584, ...).
You can use the patches later as a kind of search
archive.
Finding a start point
If you want to fix/change the behaviour of
function/feature Foo, just scan the patches for
patches that mention Foo either in the subject, the
comments, or the body of the fix. A good chance the
patch shows you the files that are affected by that
patch which are very likely to be the starting point
of your journey into the guts of perl.
Finding how to fix a bug
If you've found where the function/feature Foo mis-
behaves, but you don't know how to fix it (but you
do know the change you want to make), you can,
again, peruse the patches for similar changes and
look how others apply the fix.
Finding the source of misbehaviour
When you keep in sync with bleadperl, the pumpking
would love to see that the community efforts really
work. So after each of his sync points, you are to
'make test' to check if everything is still in work-
ing order. If it is, you do 'make ok', which will
send an OK report to perlbug@perl.org. (If you do
not have access to a mailer from the system you just
finished successfully 'make test', you can do 'make
okfile', which creates the file "perl.ok", which you
can than take to your favourite mailer and mail
yourself).
But of course, as always, things will not always
lead to a success path, and one or more test do not
pass the 'make test'. Before sending in a bug report
(using 'make nok' or 'make nokfile'), check the
mailing list if someone else has reported the bug
already and if so, confirm it by replying to that
message. If not, you might want to trace the source
of that misbehaviour before sending in the bug,
which will help all the other porters in finding the
solution.
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Here the saved patches come in very handy. You can
check the list of patches to see which patch changed
what file and what change caused the misbehaviour.
If you note that in the bug report, it saves the one
trying to solve it, looking for that point.
If searching the patches is too bothersome, you might
consider using perl's bugtron to find more information
about discussions and ramblings on posted bugs.
If you want to get the best of both worlds, rsync both
the source tree for convenience, reliability and ease
and rsync the patches for reference.
Working with the source
Because you cannot use the Perforce client, you cannot
easily generate diffs against the repository, nor will
merges occur when you update via rsync. If you edit a file
locally and then rsync against the latest source, changes
made in the remote copy will overwrite your local versions!
The best way to deal with this is to maintain a tree of sym-
links to the rsync'd source. Then, when you want to edit a
file, you remove the symlink, copy the real file into the
other tree, and edit it. You can then diff your edited file
against the original to generate a patch, and you can safely
update the original tree.
Perl's Configure script can generate this tree of symlinks
for you. The following example assumes that you have used
rsync to pull a copy of the Perl source into the perl-rsync
directory. In the directory above that one, you can execute
the following commands:
mkdir perl-dev
cd perl-dev
../perl-rsync/Configure -Dmksymlinks -Dusedevel -D"optimize=-g"
This will start the Perl configuration process. After a few
prompts, you should see something like this:
Symbolic links are supported.
Checking how to test for symbolic links...
Your builtin 'test -h' may be broken.
Trying external '/usr/bin/test -h'.
You can test for symbolic links with '/usr/bin/test -h'.
Creating the symbolic links...
(First creating the subdirectories...)
(Then creating the symlinks...)
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The specifics may vary based on your operating system, of
course. After you see this, you can abort the Configure
script, and you will see that the directory you are in has a
tree of symlinks to the perl-rsync directories and files.
If you plan to do a lot of work with the Perl source, here
are some Bourne shell script functions that can make your
life easier:
function edit {
if [ -L $1 ]; then
mv $1 $1.orig
cp $1.orig $1
vi $1
else
/bin/vi $1
fi
}
function unedit {
if [ -L $1.orig ]; then
rm $1
mv $1.orig $1
fi
}
Replace "vi" with your favorite flavor of editor.
Here is another function which will quickly generate a patch
for the files which have been edited in your symlink tree:
mkpatchorig() {
local diffopts
for f in `find . -name '*.orig' | sed s,^\./,,`
do
case `echo $f | sed 's,.orig$,,;s,.*\.,,'` in
c) diffopts=-p ;;
pod) diffopts='-F^=' ;;
*) diffopts= ;;
esac
diff -du $diffopts $f `echo $f | sed 's,.orig$,,'`
done
}
This function produces patches which include enough context
to make your changes obvious. This makes it easier for the
Perl pumpking(s) to review them when you send them to the
perl5-porters list, and that means they're more likely to
get applied.
This function assumed a GNU diff, and may require some
tweaking for other diff variants.
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Perlbug administration
There is a single remote administrative interface for modi-
fying bug status, category, open issues etc. using the RTbugtracker system, maintained by Robert Spier. Become an
administrator, and close any bugs you can get your sticky
mitts on:
http://rt.perl.org
The bugtracker mechanism for perl5 bugs in particular is at:
http://bugs6.perl.org/perlbug
To email the bug system administrators:
"perlbug-admin" <perlbug-admin@perl.org>
Submitting patches
Always submit patches to perl5-porters@perl.org. If you're
patching a core module and there's an author listed, send
the author a copy (see "Patching a core module"). This lets
other porters review your patch, which catches a surprising
number of errors in patches. Either use the diff program
(available in source code form from
ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' makepatch
(available from CPAN/authors/id/JV/). Unified diffs are
preferred, but context diffs are accepted. Do not send
RCS-style diffs or diffs without context lines. More infor-
mation is given in the Porting/patching.pod file in the Perl
source distribution. Please patch against the latest
development version (e.g., if you're fixing a bug in the
5.005 track, patch against the latest 5.005_5x version).
Only patches that survive the heat of the development branch
get applied to maintenance versions.
Your patch should update the documentation and test suite.
See "Writing a test".
To report a bug in Perl, use the program perlbug which comes
with Perl (if you can't get Perl to work, send mail to the
address perlbug@perl.org or perlbug@perl.com). Reporting
bugs through perlbug feeds into the automated bug-tracking
system, access to which is provided through the web at
http://bugs.perl.org/ . It often pays to check the archives
of the perl5-porters mailing list to see whether the bug
you're reporting has been reported before, and if so whether
it was considered a bug. See above for the location of the
searchable archives.
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The CPAN testers ( http://testers.cpan.org/ ) are a group of
volunteers who test CPAN modules on a variety of platforms.
Perl Smokers (
http://archives.develooper.com/daily-build@perl.org/ )
automatically tests Perl source releases on platforms with
various configurations. Both efforts welcome volunteers.
It's a good idea to read and lurk for a while before chip-
ping in. That way you'll get to see the dynamic of the
conversations, learn the personalities of the players, and
hopefully be better prepared to make a useful contribution
when do you speak up.
If after all this you still think you want to join the
perl5-porters mailing list, send mail to
perl5-porters-subscribe@perl.org. To unsubscribe, send mail
to perl5-porters-unsubscribe@perl.org.
To hack on the Perl guts, you'll need to read the following
things:
perlguts
This is of paramount importance, since it's the documen-
tation of what goes where in the Perl source. Read it
over a couple of times and it might start to make sense -
don't worry if it doesn't yet, because the best way to
study it is to read it in conjunction with poking at Perl
source, and we'll do that later on.
You might also want to look at Gisle Aas's illustrated
perlguts - there's no guarantee that this will be abso-
lutely up-to-date with the latest documentation in the
Perl core, but the fundamentals will be right. (
http://gisle.aas.no/perl/illguts/ )
perlxstut and perlxs
A working knowledge of XSUB programming is incredibly
useful for core hacking; XSUBs use techniques drawn from
the PP code, the portion of the guts that actually exe-
cutes a Perl program. It's a lot gentler to learn those
techniques from simple examples and explanation than from
the core itself.
perlapi
The documentation for the Perl API explains what some of
the internal functions do, as well as the many macros
used in the source.
Porting/pumpkin.pod
This is a collection of words of wisdom for a Perl
porter; some of it is only useful to the pumpkin holder,
but most of it applies to anyone wanting to go about Perl
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development.
The perl5-porters FAQ
This should be available from
http://simon-cozens.org/writings/p5p-faq ; alternatively,
you can get the FAQ emailed to you by sending mail to
"perl5-porters-faq@perl.org". It contains hints on read-
ing perl5-porters, information on how perl5-porters works
and how Perl development in general works.
Finding Your Way Around
Perl maintenance can be split into a number of areas, and
certain people (pumpkins) will have responsibility for each
area. These areas sometimes correspond to files or direc-
tories in the source kit. Among the areas are:
Core modules
Modules shipped as part of the Perl core live in the lib/
and ext/ subdirectories: lib/ is for the pure-Perl
modules, and ext/ contains the core XS modules.
Tests
There are tests for nearly all the modules, built-ins and
major bits of functionality. Test files all have a .t
suffix. Module tests live in the lib/ and ext/ direc-
tories next to the module being tested. Others live in
t/. See "Writing a test"
Documentation
Documentation maintenance includes looking after every-
thing in the pod/ directory, (as well as contributing new
documentation) and the documentation to the modules in
core.
Configure
The configure process is the way we make Perl portable
across the myriad of operating systems it supports.
Responsibility for the configure, build and installation
process, as well as the overall portability of the core
code rests with the configure pumpkin - others help out
with individual operating systems.
The files involved are the operating system directories,
(win32/, os2/, vms/ and so on) the shell scripts which
generate config.h and Makefile, as well as the metaconfig
files which generate Configure. (metaconfig isn't
included in the core distribution.)
Interpreter
And of course, there's the core of the Perl interpreter
itself. Let's have a look at that in a little more
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detail.
Before we leave looking at the layout, though, don't forget
that MANIFEST contains not only the file names in the Perl
distribution, but short descriptions of what's in them, too.
For an overview of the important files, try this:
perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
Elements of the interpreter
The work of the interpreter has two main stages: compiling
the code into the internal representation, or bytecode, and
then executing it. "Compiled code" in perlguts explains
exactly how the compilation stage happens.
Here is a short breakdown of perl's operation:
Startup
The action begins in perlmain.c. (or miniperlmain.c for
miniperl) This is very high-level code, enough to fit on
a single screen, and it resembles the code found in per-
lembed; most of the real action takes place in perl.c
First, perlmain.c allocates some memory and constructs a
Perl interpreter:
1 PERL_SYS_INIT3(&argc,&argv,&env);
2
3 if (!PL_do_undump) {
4 my_perl = perl_alloc();
5 if (!my_perl)
6 exit(1);
7 perl_construct(my_perl);
8 PL_perl_destruct_level = 0;
9 }
Line 1 is a macro, and its definition is dependent on
your operating system. Line 3 references "PL_do_undump",
a global variable - all global variables in Perl start
with "PL_". This tells you whether the current running
program was created with the "-u" flag to perl and then
undump, which means it's going to be false in any sane
context.
Line 4 calls a function in perl.c to allocate memory for
a Perl interpreter. It's quite a simple function, and the
guts of it looks like this:
my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
Here you see an example of Perl's system abstraction,
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which we'll see later: "PerlMem_malloc" is either your
system's "malloc", or Perl's own "malloc" as defined in
malloc.c if you selected that option at configure time.
Next, in line 7, we construct the interpreter; this sets
up all the special variables that Perl needs, the stacks,
and so on.
Now we pass Perl the command line options, and tell it to
go:
exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
if (!exitstatus) {
exitstatus = perl_run(my_perl);
}
"perl_parse" is actually a wrapper around "S_parse_body",
as defined in perl.c, which processes the command line
options, sets up any statically linked XS modules, opens
the program and calls "yyparse" to parse it.
Parsing
The aim of this stage is to take the Perl source, and
turn it into an op tree. We'll see what one of those
looks like later. Strictly speaking, there's three things
going on here.
"yyparse", the parser, lives in perly.c, although you're
better off reading the original YACC input in perly.y.
(Yes, Virginia, there is a YACC grammar for Perl!) The
job of the parser is to take your code and "understand"
it, splitting it into sentences, deciding which operands
go with which operators and so on.
The parser is nobly assisted by the lexer, which chunks
up your input into tokens, and decides what type of thing
each token is: a variable name, an operator, a bareword,
a subroutine, a core function, and so on. The main point
of entry to the lexer is "yylex", and that and its asso-
ciated routines can be found in toke.c. Perl isn't much
like other computer languages; it's highly context sensi-
tive at times, it can be tricky to work out what sort of
token something is, or where a token ends. As such,
there's a lot of interplay between the tokeniser and the
parser, which can get pretty frightening if you're not
used to it.
As the parser understands a Perl program, it builds up a
tree of operations for the interpreter to perform during
execution. The routines which construct and link together
the various operations are to be found in op.c, and will
be examined later.
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Optimization
Now the parsing stage is complete, and the finished tree
represents the operations that the Perl interpreter needs
to perform to execute our program. Next, Perl does a dry
run over the tree looking for optimisations: constant
expressions such as "3 + 4" will be computed now, and the
optimizer will also see if any multiple operations can be
replaced with a single one. For instance, to fetch the
variable $foo, instead of grabbing the glob *foo and
looking at the scalar component, the optimizer fiddles
the op tree to use a function which directly looks up the
scalar in question. The main optimizer is "peep" in op.c,
and many ops have their own optimizing functions.
Running
Now we're finally ready to go: we have compiled Perl byte
code, and all that's left to do is run it. The actual
execution is done by the "runops_standard" function in
run.c; more specifically, it's done by these three inno-
cent looking lines:
while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
PERL_ASYNC_CHECK();
}
You may be more comfortable with the Perl version of
that:
PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
Well, maybe not. Anyway, each op contains a function
pointer, which stipulates the function which will actu-
ally carry out the operation. This function will return
the next op in the sequence - this allows for things like
"if" which choose the next op dynamically at run time.
The "PERL_ASYNC_CHECK" makes sure that things like sig-
nals interrupt execution if required.
The actual functions called are known as PP code, and
they're spread between four files: pp_hot.c contains the
"hot" code, which is most often used and highly optim-
ized, pp_sys.c contains all the system-specific func-
tions, pp_ctl.c contains the functions which implement
control structures ("if", "while" and the like) and pp.c
contains everything else. These are, if you like, the C
code for Perl's built-in functions and operators.
Note that each "pp_" function is expected to return a
pointer to the next op. Calls to perl subs (and eval
blocks) are handled within the same runops loop, and do
not consume extra space on the C stack. For example,
"pp_entersub" and "pp_entertry" just push a "CxSUB" or
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"CxEVAL" block struct onto the context stack which con-
tain the address of the op following the sub call or
eval. They then return the first op of that sub or eval
block, and so execution continues of that sub or block.
Later, a "pp_leavesub" or "pp_leavetry" op pops the
"CxSUB" or "CxEVAL", retrieves the return op from it, and
returns it.
Exception handing
Perl's exception handing (i.e. "die" etc) is built on top
of the low-level "setjmp()"/"longjmp()" C-library func-
tions. These basically provide a way to capture the
current PC and SP registers and later restore them; i.e.
a "longjmp()" continues at the point in code where a pre-
vious "setjmp()" was done, with anything further up on
the C stack being lost. This is why code should always
save values using "SAVE_FOO" rather than in auto vari-
ables.
The perl core wraps "setjmp()" etc in the macros
"JMPENV_PUSH" and "JMPENV_JUMP". The basic rule of perl
exceptions is that "exit", and "die" (in the absence of
"eval") perform a JMPENV_JUMP(2), while "die" within
"eval" does a JMPENV_JUMP(3).
At entry points to perl, such as "perl_parse()",
"perl_run()" and "call_sv(cv, G_EVAL)" each does a
"JMPENV_PUSH", then enter a runops loop or whatever, and
handle possible exception returns. For a 2 return, final
cleanup is performed, such as popping stacks and calling
"CHECK" or "END" blocks. Amongst other things, this is
how scope cleanup still occurs during an "exit".
If a "die" can find a "CxEVAL" block on the context
stack, then the stack is popped to that level and the
return op in that block is assigned to "PL_restartop";
then a JMPENV_JUMP(3) is performed. This normally passes
control back to the guard. In the case of "perl_run" and
"call_sv", a non-null "PL_restartop" triggers re-entry to
the runops loop. The is the normal way that "die" or
"croak" is handled within an "eval".
Sometimes ops are executed within an inner runops loop,
such as tie, sort or overload code. In this case, some-
thing like
sub FETCH { eval { die } }
would cause a longjmp right back to the guard in
"perl_run", popping both runops loops, which is clearly
incorrect. One way to avoid this is for the tie code to
do a "JMPENV_PUSH" before executing "FETCH" in the inner
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runops loop, but for efficiency reasons, perl in fact
just sets a flag, using "CATCH_SET(TRUE)". The
"pp_require", "pp_entereval" and "pp_entertry" ops check
this flag, and if true, they call "docatch", which does a
"JMPENV_PUSH" and starts a new runops level to execute
the code, rather than doing it on the current loop.
As a further optimisation, on exit from the eval block in
the "FETCH", execution of the code following the block is
still carried on in the inner loop. When an exception is
raised, "docatch" compares the "JMPENV" level of the
"CxEVAL" with "PL_top_env" and if they differ, just re-
throws the exception. In this way any inner loops get
popped.
Here's an example.
1: eval { tie @a, 'A' };
2: sub A::TIEARRAY {
3: eval { die };
4: die;
5: }
To run this code, "perl_run" is called, which does a
"JMPENV_PUSH" then enters a runops loop. This loop exe-
cutes the eval and tie ops on line 1, with the eval push-
ing a "CxEVAL" onto the context stack.
The "pp_tie" does a "CATCH_SET(TRUE)", then starts a
second runops loop to execute the body of "TIEARRAY".
When it executes the entertry op on line 3, "CATCH_GET"
is true, so "pp_entertry" calls "docatch" which does a
"JMPENV_PUSH" and starts a third runops loop, which then
executes the die op. At this point the C call stack looks
like this:
Perl_pp_die
Perl_runops # third loop
S_docatch_body
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
and the context and data stacks, as shown by "-Dstv",
look like:
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STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
CX 1: EVAL => *
retop=nextstate
The die pops the first "CxEVAL" off the context stack,
sets "PL_restartop" from it, does a JMPENV_JUMP(3), and
control returns to the top "docatch". This then starts
another third-level runops level, which executes the
nextstate, pushmark and die ops on line 4. At the point
that the second "pp_die" is called, the C call stack
looks exactly like that above, even though we are no
longer within an inner eval; this is because of the
optimization mentioned earlier. However, the context
stack now looks like this, ie with the top CxEVAL popped:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
The die on line 4 pops the context stack back down to the
CxEVAL, leaving it as:
STACK 0: MAIN
CX 0: BLOCK =>
As usual, "PL_restartop" is extracted from the "CxEVAL",
and a JMPENV_JUMP(3) done, which pops the C stack back to
the docatch:
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
In this case, because the "JMPENV" level recorded in the
"CxEVAL" differs from the current one, "docatch" just
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does a JMPENV_JUMP(3) and the C stack unwinds to:
perl_run
main
Because "PL_restartop" is non-null, "run_body" starts a
new runops loop and execution continues.
Internal Variable Types
You should by now have had a look at perlguts, which tells
you about Perl's internal variable types: SVs, HVs, AVs and
the rest. If not, do that now.
These variables are used not only to represent Perl-space
variables, but also any constants in the code, as well as
some structures completely internal to Perl. The symbol
table, for instance, is an ordinary Perl hash. Your code is
represented by an SV as it's read into the parser; any pro-
gram files you call are opened via ordinary Perl filehan-
dles, and so on.
The core Devel::Peek module lets us examine SVs from a Perl
program. Let's see, for instance, how Perl treats the con-
stant "hello".
% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"\0
5 CUR = 5
6 LEN = 6
Reading "Devel::Peek" output takes a bit of practise, so
let's go through it line by line.
Line 1 tells us we're looking at an SV which lives at
0xa04ecbc in memory. SVs themselves are very simple struc-
tures, but they contain a pointer to a more complex struc-
ture. In this case, it's a PV, a structure which holds a
string value, at location 0xa041450. Line 2 is the refer-
ence count; there are no other references to this data, so
it's 1.
Line 3 are the flags for this SV - it's OK to use it as a
PV, it's a read-only SV (because it's a constant) and the
data is a PV internally. Next we've got the contents of the
string, starting at location 0xa0484e0.
Line 5 gives us the current length of the string - note that
this does not include the null terminator. Line 6 is not the
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length of the string, but the length of the currently allo-
cated buffer; as the string grows, Perl automatically
extends the available storage via a routine called "SvGROW".
You can get at any of these quantities from C very easily;
just add "Sv" to the name of the field shown in the snippet,
and you've got a macro which will return the value:
"SvCUR(sv)" returns the current length of the string,
"SvREFCOUNT(sv)" returns the reference count, "SvPV(sv,
len)" returns the string itself with its length, and so on.
More macros to manipulate these properties can be found in
perlguts.
Let's take an example of manipulating a PV, from
"sv_catpvn", in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = '\0';
13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
14 SvTAINT(sv);
15 }
This is a function which adds a string, "ptr", of length
"len" onto the end of the PV stored in "sv". The first thing
we do in line 6 is make sure that the SV has a valid PV, by
calling the "SvPV_force" macro to force a PV. As a side
effect, "tlen" gets set to the current value of the PV, and
the PV itself is returned to "junk".
In line 7, we make sure that the SV will have enough room to
accommodate the old string, the new string and the null ter-
minator. If "LEN" isn't big enough, "SvGROW" will reallocate
space for us.
Now, if "junk" is the same as the string we're trying to
add, we can grab the string directly from the SV; "SvPVX" is
the address of the PV in the SV.
Line 10 does the actual catenation: the "Move" macro moves a
chunk of memory around: we move the string "ptr" to the end
of the PV - that's the start of the PV plus its current
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length. We're moving "len" bytes of type "char". After doing
so, we need to tell Perl we've extended the string, by
altering "CUR" to reflect the new length. "SvEND" is a macro
which gives us the end of the string, so that needs to be a
"\0".
Line 13 manipulates the flags; since we've changed the PV,
any IV or NV values will no longer be valid: if we have
"$a=10; $a.="6";" we don't want to use the old IV of 10.
"SvPOK_only_utf8" is a special UTF-8-aware version of
"SvPOK_only", a macro which turns off the IOK and NOK flags
and turns on POK. The final "SvTAINT" is a macro which
launders tainted data if taint mode is turned on.
AVs and HVs are more complicated, but SVs are by far the
most common variable type being thrown around. Having seen
something of how we manipulate these, let's go on and look
at how the op tree is constructed.
Op Trees
First, what is the op tree, anyway? The op tree is the
parsed representation of your program, as we saw in our sec-
tion on parsing, and it's the sequence of operations that
Perl goes through to execute your program, as we saw in
"Running".
An op is a fundamental operation that Perl can perform: all
the built-in functions and operators are ops, and there are
a series of ops which deal with concepts the interpreter
needs internally - entering and leaving a block, ending a
statement, fetching a variable, and so on.
The op tree is connected in two ways: you can imagine that
there are two "routes" through it, two orders in which you
can traverse the tree. First, parse order reflects how the
parser understood the code, and secondly, execution order
tells perl what order to perform the operations in.
The easiest way to examine the op tree is to stop Perl after
it has finished parsing, and get it to dump out the tree.
This is exactly what the compiler backends B::Terse,
B::Concise and B::Debug do.
Let's have a look at how Perl sees "$a = $b + $c":
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% perl -MO=Terse -e '$a=$b+$c'
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
4 BINOP (0x8179828) sassign
5 BINOP (0x8179800) add [1]
6 UNOP (0x81796e0) null [15]
7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
8 UNOP (0x81797e0) null [15]
9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
10 UNOP (0x816b4f0) null [15]
11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
Let's start in the middle, at line 4. This is a BINOP, a
binary operator, which is at location 0x8179828. The
specific operator in question is "sassign" - scalar assign-
ment - and you can find the code which implements it in the
function "pp_sassign" in pp_hot.c. As a binary operator, it
has two children: the add operator, providing the result of
"$b+$c", is uppermost on line 5, and the left hand side is
on line 10.
Line 10 is the null op: this does exactly nothing. What is
that doing there? If you see the null op, it's a sign that
something has been optimized away after parsing. As we men-
tioned in "Optimization", the optimization stage sometimes
converts two operations into one, for example when fetching
a scalar variable. When this happens, instead of rewriting
the op tree and cleaning up the dangling pointers, it's
easier just to replace the redundant operation with the null
op. Originally, the tree would have looked like this:
10 SVOP (0x816b4f0) rv2sv [15]
11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
That is, fetch the "a" entry from the main symbol table, and
then look at the scalar component of it: "gvsv" ("pp_gvsv"
into pp_hot.c) happens to do both these things.
The right hand side, starting at line 5 is similar to what
we've just seen: we have the "add" op ("pp_add" also in
pp_hot.c) add together two "gvsv"s.
Now, what's this about?
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
"enter" and "leave" are scoping ops, and their job is to
perform any housekeeping every time you enter and leave a
block: lexical variables are tidied up, unreferenced
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variables are destroyed, and so on. Every program will have
those first three lines: "leave" is a list, and its children
are all the statements in the block. Statements are delim-
ited by "nextstate", so a block is a collection of "next-
state" ops, with the ops to be performed for each statement
being the children of "nextstate". "enter" is a single op
which functions as a marker.
That's how Perl parsed the program, from top to bottom:
Program
|
Statement
|
=
/ \
/ \
$a +
/ \
$b $c
However, it's impossible to perform the operations in this
order: you have to find the values of $b and $c before you
add them together, for instance. So, the other thread that
runs through the op tree is the execution order: each op has
a field "op_next" which points to the next op to be run, so
following these pointers tells us how perl executes the
code. We can traverse the tree in this order using the
"exec" option to "B::Terse":
% perl -MO=Terse,exec -e '$a=$b+$c'
1 OP (0x8179928) enter
2 COP (0x81798c8) nextstate
3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
5 BINOP (0x8179878) add [1]
6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
7 BINOP (0x81798a0) sassign
8 LISTOP (0x8179900) leave
This probably makes more sense for a human: enter a block,
start a statement. Get the values of $b and $c, and add them
together. Find $a, and assign one to the other. Then leave.
The way Perl builds up these op trees in the parsing process
can be unravelled by examining perly.y, the YACC grammar.
Let's take the piece we need to construct the tree for "$a =
$b + $c"
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1 term : term ASSIGNOP term
2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
3 | term ADDOP term
4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
If you're not used to reading BNF grammars, this is how it
works: You're fed certain things by the tokeniser, which
generally end up in upper case. Here, "ADDOP", is provided
when the tokeniser sees "+" in your code. "ASSIGNOP" is pro-
vided when "=" is used for assigning. These are "terminal
symbols", because you can't get any simpler than them.
The grammar, lines one and three of the snippet above, tells
you how to build up more complex forms. These complex forms,
"non-terminal symbols" are generally placed in lower case.
"term" here is a non-terminal symbol, representing a single
expression.
The grammar gives you the following rule: you can make the
thing on the left of the colon if you see all the things on
the right in sequence. This is called a "reduction", and the
aim of parsing is to completely reduce the input. There are
several different ways you can perform a reduction,
separated by vertical bars: so, "term" followed by "=" fol-
lowed by "term" makes a "term", and "term" followed by "+"
followed by "term" can also make a "term".
So, if you see two terms with an "=" or "+", between them,
you can turn them into a single expression. When you do
this, you execute the code in the block on the next line: if
you see "=", you'll do the code in line 2. If you see "+",
you'll do the code in line 4. It's this code which contri-
butes to the op tree.
| term ADDOP term
{ $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
What this does is creates a new binary op, and feeds it a
number of variables. The variables refer to the tokens: $1
is the first token in the input, $2 the second, and so on -
think regular expression backreferences. $$ is the op
returned from this reduction. So, we call "newBINOP" to
create a new binary operator. The first parameter to "newBI-
NOP", a function in op.c, is the op type. It's an addition
operator, so we want the type to be "ADDOP". We could
specify this directly, but it's right there as the second
token in the input, so we use $2. The second parameter is
the op's flags: 0 means "nothing special". Then the things
to add: the left and right hand side of our expression, in
scalar context.
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Stacks
When perl executes something like "addop", how does it pass
on its results to the next op? The answer is, through the
use of stacks. Perl has a number of stacks to store things
it's currently working on, and we'll look at the three most
important ones here.
Argument stack
Arguments are passed to PP code and returned from PP code
using the argument stack, "ST". The typical way to handle
arguments is to pop them off the stack, deal with them
how you wish, and then push the result back onto the
stack. This is how, for instance, the cosine operator
works:
NV value;
value = POPn;
value = Perl_cos(value);
XPUSHn(value);
We'll see a more tricky example of this when we consider
Perl's macros below. "POPn" gives you the NV (floating
point value) of the top SV on the stack: the $x in
"cos($x)". Then we compute the cosine, and push the
result back as an NV. The "X" in "XPUSHn" means that the
stack should be extended if necessary - it can't be
necessary here, because we know there's room for one more
item on the stack, since we've just removed one! The
"XPUSH*" macros at least guarantee safety.
Alternatively, you can fiddle with the stack directly:
"SP" gives you the first element in your portion of the
stack, and "TOP*" gives you the top SV/IV/NV/etc. on the
stack. So, for instance, to do unary negation of an
integer:
SETi(-TOPi);
Just set the integer value of the top stack entry to its
negation.
Argument stack manipulation in the core is exactly the
same as it is in XSUBs - see perlxstut, perlxs and perl-
guts for a longer description of the macros used in stack
manipulation.
Mark stack
I say "your portion of the stack" above because PP code
doesn't necessarily get the whole stack to itself: if
your function calls another function, you'll only want to
expose the arguments aimed for the called function, and
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not (necessarily) let it get at your own data. The way we
do this is to have a "virtual" bottom-of-stack, exposed
to each function. The mark stack keeps bookmarks to loca-
tions in the argument stack usable by each function. For
instance, when dealing with a tied variable, (internally,
something with "P" magic) Perl has to call methods for
accesses to the tied variables. However, we need to
separate the arguments exposed to the method to the argu-
ment exposed to the original function - the store or
fetch or whatever it may be. Here's how the tied "push"
is implemented; see "av_push" in av.c:
1 PUSHMARK(SP);
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
5 PUTBACK;
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
9 POPSTACK;
The lines which concern the mark stack are the first,
fifth and last lines: they save away, restore and remove
the current position of the argument stack.
Let's examine the whole implementation, for practice:
1 PUSHMARK(SP);
Push the current state of the stack pointer onto the mark
stack. This is so that when we've finished adding items
to the argument stack, Perl knows how many things we've
added recently.
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
We're going to add two more items onto the argument
stack: when you have a tied array, the "PUSH" subroutine
receives the object and the value to be pushed, and
that's exactly what we have here - the tied object,
retrieved with "SvTIED_obj", and the value, the SV "val".
5 PUTBACK;
Next we tell Perl to make the change to the global stack
pointer: "dSP" only gave us a local copy, not a reference
to the global.
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6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
"ENTER" and "LEAVE" localise a block of code - they make
sure that all variables are tidied up, everything that
has been localised gets its previous value returned, and
so on. Think of them as the "{" and "}" of a Perl block.
To actually do the magic method call, we have to call a
subroutine in Perl space: "call_method" takes care of
that, and it's described in perlcall. We call the "PUSH"
method in scalar context, and we're going to discard its
return value.
9 POPSTACK;
Finally, we remove the value we placed on the mark stack,
since we don't need it any more.
Save stack
C doesn't have a concept of local scope, so perl provides
one. We've seen that "ENTER" and "LEAVE" are used as
scoping braces; the save stack implements the C
equivalent of, for example:
{
local $foo = 42;
...
}
See "Localising Changes" in perlguts for how to use the
save stack.
Millions of Macros
One thing you'll notice about the Perl source is that it's
full of macros. Some have called the pervasive use of macros
the hardest thing to understand, others find it adds to
clarity. Let's take an example, the code which implements
the addition operator:
1 PP(pp_add)
2 {
3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
4 {
5 dPOPTOPnnrl_ul;
6 SETn( left + right );
7 RETURN;
8 }
9 }
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Every line here (apart from the braces, of course) contains
a macro. The first line sets up the function declaration as
Perl expects for PP code; line 3 sets up variable declara-
tions for the argument stack and the target, the return
value of the operation. Finally, it tries to see if the
addition operation is overloaded; if so, the appropriate
subroutine is called.
Line 5 is another variable declaration - all variable
declarations start with "d" - which pops from the top of the
argument stack two NVs (hence "nn") and puts them into the
variables "right" and "left", hence the "rl". These are the
two operands to the addition operator. Next, we call "SETn"
to set the NV of the return value to the result of adding
the two values. This done, we return - the "RETURN" macro
makes sure that our return value is properly handled, and we
pass the next operator to run back to the main run loop.
Most of these macros are explained in perlapi, and some of
the more important ones are explained in perlxs as well. Pay
special attention to "Background and PERL_IMPLICIT_CONTEXT"
in perlguts for information on the "[pad]THX_?" macros.
The .i Targets
You can expand the macros in a foo.c file by saying
make foo.i
which will expand the macros using cpp. Don't be scared by
the results.
Poking at Perl
To really poke around with Perl, you'll probably want to
build Perl for debugging, like this:
./Configure -d -D optimize=-g
make
"-g" is a flag to the C compiler to have it produce debug-
ging information which will allow us to step through a run-
ning program. Configure will also turn on the "DEBUGGING"
compilation symbol which enables all the internal debugging
code in Perl. There are a whole bunch of things you can
debug with this: perlrun lists them all, and the best way to
find out about them is to play about with them. The most
useful options are probably
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l Context (loop) stack processing
t Trace execution
o Method and overloading resolution
c String/numeric conversions
Some of the functionality of the debugging code can be
achieved using XS modules.
-Dr => use re 'debug'
-Dx => use O 'Debug'
Using a source-level debugger
If the debugging output of "-D" doesn't help you, it's time
to step through perl's execution with a source-level
debugger.
+ We'll use "gdb" for our examples here; the principles
will apply to any debugger, but check the manual of the
one you're using.
To fire up the debugger, type
gdb ./perl
You'll want to do that in your Perl source tree so the
debugger can read the source code. You should see the copy-
right message, followed by the prompt.
(gdb)
"help" will get you into the documentation, but here are the
most useful commands:
run [args]
Run the program with the given arguments.
break function_name
break source.c:xxx
Tells the debugger that we'll want to pause execution
when we reach either the named function (but see "Inter-
nal Functions" in perlguts!) or the given line in the
named source file.
step
Steps through the program a line at a time.
next
Steps through the program a line at a time, without des-
cending into functions.
continue
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Run until the next breakpoint.
finish
Run until the end of the current function, then stop
again.
'enter'
Just pressing Enter will do the most recent operation
again - it's a blessing when stepping through miles of
source code.
print
Execute the given C code and print its results. WARNING:
Perl makes heavy use of macros, and gdb does not neces-
sarily support macros (see later "gdb macro support").
You'll have to substitute them yourself, or to invoke cpp
on the source code files (see "The .i Targets") So, for
instance, you can't say
print SvPV_nolen(sv)
but you have to say
print Perl_sv_2pv_nolen(sv)
You may find it helpful to have a "macro dictionary", which
you can produce by saying "cpp -dM perl.c | sort". Even
then, cpp won't recursively apply those macros for you.
gdb macro support
Recent versions of gdb have fairly good macro support, but
in order to use it you'll need to compile perl with macro
definitions included in the debugging information. Using
gcc version 3.1, this means configuring with
"-Doptimize=-g3". Other compilers might use a different
switch (if they support debugging macros at all).
Dumping Perl Data Structures
One way to get around this macro hell is to use the dumping
functions in dump.c; these work a little like an internal
Devel::Peek, but they also cover OPs and other structures
that you can't get at from Perl. Let's take an example.
We'll use the "$a = $b + $c" we used before, but give it a
bit of context: "$b = "6XXXX"; $c = 2.3;". Where's a good
place to stop and poke around?
What about "pp_add", the function we examined earlier to
implement the "+" operator:
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(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use "Perl_pp_add" and not "pp_add" - see "Internal
Functions" in perlguts. With the breakpoint in place, we can
run our program:
(gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
Lots of junk will go past as gdb reads in the relevant
source files and libraries, and then:
Breakpoint 1, Perl_pp_add () at pp_hot.c:309
309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
(gdb) step
311 dPOPTOPnnrl_ul;
(gdb)
We looked at this bit of code before, and we said that
"dPOPTOPnnrl_ul" arranges for two "NV"s to be placed into
"left" and "right" - let's slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \
SV *leftsv = TOPs; \
NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
"POPn" takes the SV from the top of the stack and obtains
its NV either directly (if "SvNOK" is set) or by calling the
"sv_2nv" function. "TOPs" takes the next SV from the top of
the stack - yes, "POPn" uses "TOPs" - but doesn't remove it.
We then use "SvNV" to get the NV from "leftsv" in the same
way as before - yes, "POPn" uses "SvNV".
Since we don't have an NV for $b, we'll have to use "sv_2nv"
to convert it. If we step again, we'll find ourselves there:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(gdb)
We can now use "Perl_sv_dump" to investigate the SV:
SV = PV(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"\0
CUR = 5
LEN = 6
$1 = void
We know we're going to get 6 from this, so let's finish the
subroutine:
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(gdb) finish
Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
0x462669 in Perl_pp_add () at pp_hot.c:311
311 dPOPTOPnnrl_ul;
We can also dump out this op: the current op is always
stored in "PL_op", and we can dump it with "Perl_op_dump".
This'll give us similar output to B::Debug.
{
13 TYPE = add ===> 14
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (12)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
11 TYPE = gvsv ===> 12
FLAGS = (SCALAR)
GV = main::b
}
}
# finish this later #
Patching
All right, we've now had a look at how to navigate the Perl
sources and some things you'll need to know when fiddling
with them. Let's now get on and create a simple patch.
Here's something Larry suggested: if a "U" is the first
active format during a "pack", (for example, "pack "U3C8",
@stuff") then the resulting string should be treated as
UTF-8 encoded.
How do we prepare to fix this up? First we locate the code
in question - the "pack" happens at runtime, so it's going
to be in one of the pp files. Sure enough, "pp_pack" is in
pp.c. Since we're going to be altering this file, let's copy
it to pp.c~.
[Well, it was in pp.c when this tutorial was written. It has
now been split off with "pp_unpack" to its own file,
pp_pack.c]
Now let's look over "pp_pack": we take a pattern into "pat",
and then loop over the pattern, taking each format character
in turn into "datum_type". Then for each possible format
character, we swallow up the other arguments in the pattern
(a field width, an asterisk, and so on) and convert the next
chunk input into the specified format, adding it onto the
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output SV "cat".
How do we know if the "U" is the first format in the "pat"?
Well, if we have a pointer to the start of "pat" then, if we
see a "U" we can test whether we're still at the start of
the string. So, here's where "pat" is set up:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
SV *fromstr;
We'll have another string pointer in there:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
+ char *patcopy;
register I32 len;
I32 datumtype;
SV *fromstr;
And just before we start the loop, we'll set "patcopy" to be
the start of "pat":
items = SP - MARK;
MARK++;
sv_setpvn(cat, "", 0);
+ patcopy = pat;
while (pat < patend) {
Now if we see a "U" which was at the start of the string, we
turn on the "UTF8" flag for the output SV, "cat":
+ if (datumtype == 'U' && pat==patcopy+1)
+ SvUTF8_on(cat);
if (datumtype == '#') {
while (pat < patend && *pat != '\n')
pat++;
Remember that it has to be "patcopy+1" because the first
character of the string is the "U" which has been swallowed
into "datumtype!"
Oops, we forgot one thing: what if there are spaces at the
start of the pattern? "pack(" U*", @stuff)" will have "U"
as the first active character, even though it's not the
first thing in the pattern. In this case, we have to advance
"patcopy" along with "pat" when we see spaces:
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if (isSPACE(datumtype))
continue;
needs to become
if (isSPACE(datumtype)) {
patcopy++;
continue;
}
OK. That's the C part done. Now we must do two additional
things before this patch is ready to go: we've changed the
behaviour of Perl, and so we must document that change. We
must also provide some more regression tests to make sure
our patch works and doesn't create a bug somewhere else
along the line.
The regression tests for each operator live in t/op/, and so
we make a copy of t/op/pack.t to t/op/pack.t~. Now we can
add our tests to the end. First, we'll test that the "U"
does indeed create Unicode strings.
t/op/pack.t has a sensible ok() function, but if it didn't
we could use the one from t/test.pl.
require './test.pl';
plan( tests => 159 );
so instead of this:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000);
print "ok $test\n"; $test++;
we can write the more sensible (see Test::More for a full
explanation of is() and other testing functions).
is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
"U* produces unicode" );
Now we'll test that we got that space-at-the-beginning busi-
ness right:
is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
" with spaces at the beginning" );
And finally we'll test that we don't make Unicode strings if
"U" is not the first active format:
isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
"U* not first isn't unicode" );
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Mustn't forget to change the number of tests which appears
at the top, or else the automated tester will get confused.
This will either look like this:
print "1..156\n";
or this:
plan( tests => 156 );
We now compile up Perl, and run it through the test suite.
Our new tests pass, hooray!
Finally, the documentation. The job is never done until the
paperwork is over, so let's describe the change we've just
made. The relevant place is pod/perlfunc.pod; again, we make
a copy, and then we'll insert this text in the description
of "pack":
=item *
If the pattern begins with a C<U>, the resulting string will be treated
as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
with an initial C<U0>, and the bytes that follow will be interpreted as
Unicode characters. If you don't want this to happen, you can begin your
pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your
string, and then follow this with a C<U*> somewhere in your pattern.
All done. Now let's create the patch. Porting/patching.pod
tells us that if we're making major changes, we should copy
the entire directory to somewhere safe before we begin fid-
dling, and then do
diff -ruN old new > patch
However, we know which files we've changed, and we can sim-
ply do this:
diff -u pp.c~ pp.c > patch
diff -u t/op/pack.t~ t/op/pack.t >> patch
diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch
We end up with a patch looking a little like this:
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--- pp.c~ Fri Jun 02 04:34:10 2000
+++ pp.c Fri Jun 16 11:37:25 2000
@@ -4375,6 +4375,7 @@
register I32 items;
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
+ char *patcopy;
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
@@ -4405,6 +4406,7 @@
...
And finally, we submit it, with our rationale, to
perl5-porters. Job done!
Patching a core module
This works just like patching anything else, with an extra
consideration. Many core modules also live on CPAN. If
this is so, patch the CPAN version instead of the core and
send the patch off to the module maintainer (with a copy to
p5p). This will help the module maintainer keep the CPAN
version in sync with the core version without constantly
scanning p5p.
The list of maintainers of core modules is usefully docu-
mented in Porting/Maintainers.pl.
Adding a new function to the core
If, as part of a patch to fix a bug, or just because you
have an especially good idea, you decide to add a new func-
tion to the core, discuss your ideas on p5p well before you
start work. It may be that someone else has already
attempted to do what you are considering and can give lots
of good advice or even provide you with bits of code that
they already started (but never finished).
You have to follow all of the advice given above for patch-
ing. It is extremely important to test any addition
thoroughly and add new tests to explore all boundary condi-
tions that your new function is expected to handle. If your
new function is used only by one module (e.g. toke), then it
should probably be named S_your_function (for static); on
the other hand, if you expect it to accessible from other
functions in Perl, you should name it Perl_your_function.
See "Internal Functions" in perlguts for more details.
The location of any new code is also an important considera-
tion. Don't just create a new top level .c file and put
your code there; you would have to make changes to Configure
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(so the Makefile is created properly), as well as possibly
lots of include files. This is strictly pumpking business.
It is better to add your function to one of the existing top
level source code files, but your choice is complicated by
the nature of the Perl distribution. Only the files that
are marked as compiled static are located in the perl exe-
cutable. Everything else is located in the shared library
(or DLL if you are running under WIN32). So, for example,
if a function was only used by functions located in toke.c,
then your code can go in toke.c. If, however, you want to
call the function from universal.c, then you should put your
code in another location, for example util.c.
In addition to writing your c-code, you will need to create
an appropriate entry in embed.pl describing your function,
then run 'make regen_headers' to create the entries in the
numerous header files that perl needs to compile correctly.
See "Internal Functions" in perlguts for information on the
various options that you can set in embed.pl. You will for-
get to do this a few (or many) times and you will get warn-
ings during the compilation phase. Make sure that you men-
tion this when you post your patch to P5P; the pumpking
needs to know this.
When you write your new code, please be conscious of exist-
ing code conventions used in the perl source files. See
perlstyle for details. Although most of the guidelines dis-
cussed seem to focus on Perl code, rather than c, they all
apply (except when they don't ;). See also
Porting/patching.pod file in the Perl source distribution
for lots of details about both formatting and submitting
patches of your changes.
Lastly, TEST TEST TEST TEST TEST any code before posting to
p5p. Test on as many platforms as you can find. Test as
many perl Configure options as you can (e.g. MULTIPLICITY).
If you have profiling or memory tools, see "EXTERNAL TOOLS
FOR DEBUGGING PERL" below for how to use them to further
test your code. Remember that most of the people on P5P are
doing this on their own time and don't have the time to
debug your code.
Writing a test
Every module and built-in function has an associated test
file (or should...). If you add or change functionality,
you have to write a test. If you fix a bug, you have to
write a test so that bug never comes back. If you alter the
docs, it would be nice to test what the new documentation
says.
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In short, if you submit a patch you probably also have to
patch the tests.
For modules, the test file is right next to the module
itself. lib/strict.t tests lib/strict.pm. This is a recent
innovation, so there are some snags (and it would be wonder-
ful for you to brush them out), but it basically works that
way. Everything else lives in t/.
t/base/
Testing of the absolute basic functionality of Perl.
Things like "if", basic file reads and writes, simple
regexes, etc. These are run first in the test suite and
if any of them fail, something is really broken.
t/cmd/
These test the basic control structures, "if/else",
"while", subroutines, etc.
t/comp/
Tests basic issues of how Perl parses and compiles
itself.
t/io/
Tests for built-in IO functions, including command line
arguments.
t/lib/
The old home for the module tests, you shouldn't put any-
thing new in here. There are still some bits and pieces
hanging around in here that need to be moved. Perhaps
you could move them? Thanks!
t/op/
Tests for perl's built in functions that don't fit into
any of the other directories.
t/pod/
Tests for POD directives. There are still some tests for
the Pod modules hanging around in here that need to be
moved out into lib/.
t/run/
Testing features of how perl actually runs, including
exit codes and handling of PERL* environment variables.
t/uni/
Tests for the core support of Unicode.
t/win32/
Windows-specific tests.
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t/x2p
A test suite for the s2p converter.
The core uses the same testing style as the rest of Perl, a
simple "ok/not ok" run through Test::Harness, but there are
a few special considerations.
There are three ways to write a test in the core.
Test::More, t/test.pl and ad hoc "print $test ? "ok 42\n" :
"not ok 42\n"". The decision of which to use depends on
what part of the test suite you're working on. This is a
measure to prevent a high-level failure (such as Config.pm
breaking) from causing basic functionality tests to fail.
t/base t/comp
Since we don't know if require works, or even subrou-
tines, use ad hoc tests for these two. Step carefully
to avoid using the feature being tested.
t/cmd t/run t/io t/op
Now that basic require() and subroutines are tested, you
can use the t/test.pl library which emulates the impor-
tant features of Test::More while using a minimum of
core features.
You can also conditionally use certain libraries like
Config, but be sure to skip the test gracefully if it's
not there.
t/lib ext lib
Now that the core of Perl is tested, Test::More can be
used. You can also use the full suite of core modules
in the tests.
When you say "make test" Perl uses the t/TEST program to run
the test suite (except under Win32 where it uses t/harness
instead.) All tests are run from the t/ directory, not the
directory which contains the test. This causes some prob-
lems with the tests in lib/, so here's some opportunity for
some patching.
You must be triply conscious of cross-platform concerns.
This usually boils down to using File::Spec and avoiding
things like "fork()" and "system()" unless absolutely neces-
sary.
Special Make Test Targets
There are various special make targets that can be used to
test Perl slightly differently than the standard "test" tar-
get. Not all them are expected to give a 100% success rate.
Many of them have several aliases, and many of them are not
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available on certain operating systems.
coretest
Run perl on all core tests (t/* and lib/[a-z]* pragma
tests).
(Not available on Win32)
test.deparse
Run all the tests through B::Deparse. Not all tests
will succeed.
(Not available on Win32)
test.taintwarn
Run all tests with the -t command-line switch. Not all
tests are expected to succeed (until they're specifi-
cally fixed, of course).
(Not available on Win32)
minitest
Run miniperl on t/base, t/comp, t/cmd, t/run, t/io,
t/op, and t/uni tests.
test.valgrind check.valgrind utest.valgrind ucheck.valgrind
(Only in Linux) Run all the tests using the memory leak
+ naughty memory access tool "valgrind". The log files
will be named testname.valgrind.
test.third check.third utest.third ucheck.third
(Only in Tru64) Run all the tests using the memory leak
+ naughty memory access tool "Third Degree". The log
files will be named perl.3log.testname.
test.torture torturetest
Run all the usual tests and some extra tests. As of
Perl 5.8.0 the only extra tests are Abigail's JAPHs,
t/japh/abigail.t.
You can also run the torture test with t/harness by giv-
ing "-torture" argument to t/harness.
utest ucheck test.utf8 check.utf8
Run all the tests with -Mutf8. Not all tests will
succeed.
(Not available on Win32)
minitest.utf16 test.utf16
Runs the tests with UTF-16 encoded scripts, encoded with
different versions of this encoding.
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"make utest.utf16" runs the test suite with a combina-
tion of "-utf8" and "-utf16" arguments to t/TEST.
(Not available on Win32)
test_harness
Run the test suite with the t/harness controlling pro-
gram, instead of t/TEST. t/harness is more sophisti-
cated, and uses the Test::Harness module, thus using
this test target supposes that perl mostly works. The
main advantage for our purposes is that it prints a
detailed summary of failed tests at the end. Also,
unlike t/TEST, it doesn't redirect stderr to stdout.
Note that under Win32 t/harness is always used instead
of t/TEST, so there is no special "test_harness" target.
Under Win32's "test" target you may use the
TEST_SWITCHES and TEST_FILES environment variables to
control the behaviour of t/harness. This means you can
say
nmake test TEST_FILES="op/*.t"
nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
test-notty test_notty
Sets PERL_SKIP_TTY_TEST to true before running normal
test.
Running tests by hand
You can run part of the test suite by hand by using one the
following commands from the t/ directory :
./perl -I../lib TEST list-of-.t-files
or
./perl -I../lib harness list-of-.t-files
(if you don't specify test scripts, the whole test suite
will be run.)
Using t/harness for testing
If you use "harness" for testing you have several command
line options available to you. The arguments are as follows,
and are in the order that they must appear if used together.
harness -v -torture -re=pattern LIST OF FILES TO TEST
harness -v -torture -re LIST OF PATTERNS TO MATCH
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If "LIST OF FILES TO TEST" is omitted the file list is
obtained from the manifest. The file list may include shell
wildcards which will be expanded out.
-v Run the tests under verbose mode so you can see what
tests were run, and debug outbut.
-torture
Run the torture tests as well as the normal set.
-re=PATTERN
Filter the file list so that all the test files run
match PATTERN. Note that this form is distinct from the
-re LIST OF PATTERNS form below in that it allows the
file list to be provided as well.
-re LIST OF PATTERNS
Filter the file list so that all the test files run
match /(LIST|OF|PATTERNS)/. Note that with this form the
patterns are joined by '|' and you cannot supply a list
of files, instead the test files are obtained from the
MANIFEST.
You can run an individual test by a command similar to
./perl -I../lib patho/to/foo.t
except that the harnesses set up some environment variables
that may affect the execution of the test :
PERL_CORE=1
indicates that we're running this test part of the perl
core test suite. This is useful for modules that have a
dual life on CPAN.
PERL_DESTRUCT_LEVEL=2
is set to 2 if it isn't set already (see
"PERL_DESTRUCT_LEVEL")
PERL
(used only by t/TEST) if set, overrides the path to the
perl executable that should be used to run the tests
(the default being ./perl).
PERL_SKIP_TTY_TEST
if set, tells to skip the tests that need a terminal.
It's actually set automatically by the Makefile, but can
also be forced artificially by running 'make
test_notty'.

EXTERNAL TOOLS FOR DEBUGGING PERL

Sometimes it helps to use external tools while debugging and
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testing Perl. This section tries to guide you through using
some common testing and debugging tools with Perl. This is
meant as a guide to interfacing these tools with Perl, not
as any kind of guide to the use of the tools themselves.
NOTE 1: Running under memory debuggers such as Purify, val-
grind, or Third Degree greatly slows down the execution:
seconds become minutes, minutes become hours. For example
as of Perl 5.8.1, the ext/Encode/t/Unicode.t takes extraor-
dinarily long to complete under e.g. Purify, Third Degree,
and valgrind. Under valgrind it takes more than six hours,
even on a snappy computer-- the said test must be doing
something that is quite unfriendly for memory debuggers. If
you don't feel like waiting, that you can simply kill away
the perl process.
NOTE 2: To minimize the number of memory leak false alarms
(see "PERL_DESTRUCT_LEVEL" for more information), you have
to have environment variable PERL_DESTRUCT_LEVEL set to 2.
The TEST and harness scripts do that automatically. But if
you are running some of the tests manually-- for csh-like
shells:
setenv PERL_DESTRUCT_LEVEL 2
and for Bourne-type shells:
PERL_DESTRUCT_LEVEL=2
export PERL_DESTRUCT_LEVEL
or in UNIXy environments you can also use the "env" command:
env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
NOTE 3: There are known memory leaks when there are
compile-time errors within eval or require, seeing
"S_doeval" in the call stack is a good sign of these. Fix-
ing these leaks is non-trivial, unfortunately, but they must
be fixed eventually.
Rational Software's Purify
Purify is a commercial tool that is helpful in identifying
memory overruns, wild pointers, memory leaks and other such
badness. Perl must be compiled in a specific way for
optimal testing with Purify. Purify is available under Win-
dows NT, Solaris, HP-UX, SGI, and Siemens Unix.
Purify on Unix
On Unix, Purify creates a new Perl binary. To get the most
benefit out of Purify, you should create the perl to Purify
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using:
sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
-Uusemymalloc -Dusemultiplicity
where these arguments mean:
-Accflags=-DPURIFY
Disables Perl's arena memory allocation functions, as
well as forcing use of memory allocation functions
derived from the system malloc.
-Doptimize='-g'
Adds debugging information so that you see the exact
source statements where the problem occurs. Without
this flag, all you will see is the source filename of
where the error occurred.
-Uusemymalloc
Disable Perl's malloc so that Purify can more closely
monitor allocations and leaks. Using Perl's malloc will
make Purify report most leaks in the "potential" leaks
category.
-Dusemultiplicity
Enabling the multiplicity option allows perl to clean up
thoroughly when the interpreter shuts down, which
reduces the number of bogus leak reports from Purify.
Once you've compiled a perl suitable for Purify'ing, then
you can just:
make pureperl
which creates a binary named 'pureperl' that has been
Purify'ed. This binary is used in place of the standard
'perl' binary when you want to debug Perl memory problems.
As an example, to show any memory leaks produced during the
standard Perl testset you would create and run the Purify'ed
perl as:
make pureperl
cd t
../pureperl -I../lib harness
which would run Perl on test.pl and report any memory prob-
lems.
Purify outputs messages in "Viewer" windows by default. If
you don't have a windowing environment or if you simply want
the Purify output to unobtrusively go to a log file instead
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of to the interactive window, use these following options to
output to the log file "perl.log":
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
-log-file=perl.log -append-logfile=yes"
If you plan to use the "Viewer" windows, then you only need
this option:
setenv PURIFYOPTIONS "-chain-length=25"
In Bourne-type shells:
PURIFYOPTIONS="..."
export PURIFYOPTIONS
or if you have the "env" utility:
env PURIFYOPTIONS="..." ../pureperl ...
Purify on NT
Purify on Windows NT instruments the Perl binary 'perl.exe'
on the fly. There are several options in the makefile you
should change to get the most use out of Purify:
DEFINES
You should add -DPURIFY to the DEFINES line so the
DEFINES line looks something like:
DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
to disable Perl's arena memory allocation functions, as
well as to force use of memory allocation functions
derived from the system malloc.
USE_MULTI = define
Enabling the multiplicity option allows perl to clean up
thoroughly when the interpreter shuts down, which
reduces the number of bogus leak reports from Purify.
#PERL_MALLOC = define
Disable Perl's malloc so that Purify can more closely
monitor allocations and leaks. Using Perl's malloc will
make Purify report most leaks in the "potential" leaks
category.
CFG = Debug
Adds debugging information so that you see the exact
source statements where the problem occurs. Without
this flag, all you will see is the source filename of
where the error occurred.
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As an example, to show any memory leaks produced during the
standard Perl testset you would create and run Purify as:
cd win32
make
cd ../t
purify ../perl -I../lib harness
which would instrument Perl in memory, run Perl on test.pl,
then finally report any memory problems.
valgrind
The excellent valgrind tool can be used to find out both
memory leaks and illegal memory accesses. As of August 2003
it unfortunately works only on x86 (ELF) Linux. The special
"test.valgrind" target can be used to run the tests under
valgrind. Found errors and memory leaks are logged in files
named test.valgrind.
As system libraries (most notably glibc) are also triggering
errors, valgrind allows to suppress such errors using
suppression files. The default suppression file that comes
with valgrind already catches a lot of them. Some additional
suppressions are defined in t/perl.supp.
To get valgrind and for more information see
http://developer.kde.org/~sewardj/
Compaq's/Digital's/HP's Third Degree
Third Degree is a tool for memory leak detection and memory
access checks. It is one of the many tools in the ATOM
toolkit. The toolkit is only available on Tru64 (formerly
known as Digital UNIX formerly known as DEC OSF/1).
When building Perl, you must first run Configure with
-Doptimize=-g and -Uusemymalloc flags, after that you can
use the make targets "perl.third" and "test.third". (What
is required is that Perl must be compiled using the "-g"
flag, you may need to re-Configure.)
The short story is that with "atom" you can instrument the
Perl executable to create a new executable called
perl.third. When the instrumented executable is run, it
creates a log of dubious memory traffic in file called
perl.3log. See the manual pages of atom and third for more
information. The most extensive Third Degree documentation
is available in the Compaq "Tru64 UNIX Programmer's Guide",
chapter "Debugging Programs with Third Degree".
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The "test.third" leaves a lot of files named foo_bar.3log in
the t/ subdirectory. There is a problem with these files:
Third Degree is so effective that it finds problems also in
the system libraries. Therefore you should used the
Porting/thirdclean script to cleanup the *.3log files.
There are also leaks that for given certain definition of a
leak, aren't. See "PERL_DESTRUCT_LEVEL" for more informa-
tion.
PERL_DESTRUCT_LEVEL
If you want to run any of the tests yourself manually using
e.g. valgrind, or the pureperl or perl.third executables,
please note that by default perl does not explicitly cleanup
all the memory it has allocated (such as global memory are-
nas) but instead lets the exit() of the whole program "take
care" of such allocations, also known as "global destruction
of objects".
There is a way to tell perl to do complete cleanup: set the
environment variable PERL_DESTRUCT_LEVEL to a non-zero
value. The t/TEST wrapper does set this to 2, and this is
what you need to do too, if you don't want to see the "glo-
bal leaks": For example, for "third-degreed" Perl:
env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
(Note: the mod_perl apache module uses also this environment
variable for its own purposes and extended its semantics.
Refer to the mod_perl documentation for more information.
Also, spawned threads do the equivalent of setting this
variable to the value 1.)
If, at the end of a run you get the message N scalarsleaked, you can recompile with "-DDEBUG_LEAKING_SCALARS",
which will cause the addresses of all those leaked SVs to be
dumped; it also converts "new_SV()" from a macro into a real
function, so you can use your favourite debugger to discover
where those pesky SVs were allocated.
Profiling
Depending on your platform there are various of profiling
Perl.
There are two commonly used techniques of profiling execut-
ables: statistical time-sampling and basic-block counting.
The first method takes periodically samples of the CPU pro-
gram counter, and since the program counter can be corre-
lated with the code generated for functions, we get a
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statistical view of in which functions the program is spend-
ing its time. The caveats are that very small/fast func-
tions have lower probability of showing up in the profile,
and that periodically interrupting the program (this is usu-
ally done rather frequently, in the scale of milliseconds)
imposes an additional overhead that may skew the results.
The first problem can be alleviated by running the code for
longer (in general this is a good idea for profiling), the
second problem is usually kept in guard by the profiling
tools themselves.
The second method divides up the generated code into basicblocks. Basic blocks are sections of code that are entered
only in the beginning and exited only at the end. For exam-
ple, a conditional jump starts a basic block. Basic block
profiling usually works by instrumenting the code by adding
enter basic block #nnnn book-keeping code to the generated
code. During the execution of the code the basic block
counters are then updated appropriately. The caveat is that
the added extra code can skew the results: again, the pro-
filing tools usually try to factor their own effects out of
the results.
Gprof Profiling
gprof is a profiling tool available in many UNIX platforms,
it uses statistical time-sampling.
You can build a profiled version of perl called "perl.gprof"
by invoking the make target "perl.gprof" (What is required
is that Perl must be compiled using the "-pg" flag, you may
need to re-Configure). Running the profiled version of Perl
will create an output file called gmon.out is created which
contains the profiling data collected during the execution.
The gprof tool can then display the collected data in vari-
ous ways. Usually gprof understands the following options:
-a Suppress statically defined functions from the profile.
-b Suppress the verbose descriptions in the profile.
-e routine
Exclude the given routine and its descendants from the
profile.
-f routine
Display only the given routine and its descendants in
the profile.
-s Generate a summary file called gmon.sum which then may
be given to subsequent gprof runs to accumulate data
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over several runs.
-z Display routines that have zero usage.
For more detailed explanation of the available commands and
output formats, see your own local documentation of gprof.
GCC gcov Profiling
Starting from GCC 3.0 basic block profiling is officially
available for the GNU CC.
You can build a profiled version of perl called perl.gcov by
invoking the make target "perl.gcov" (what is required that
Perl must be compiled using gcc with the flags
"-fprofile-arcs -ftest-coverage", you may need to
re-Configure).
Running the profiled version of Perl will cause profile out-
put to be generated. For each source file an accompanying
".da" file will be created.
To display the results you use the "gcov" utility (which
should be installed if you have gcc 3.0 or newer installed).
gcov is run on source code files, like this
gcov sv.c
which will cause sv.c.gcov to be created. The .gcov files
contain the source code annotated with relative frequencies
of execution indicated by "#" markers.
Useful options of gcov include "-b" which will summarise the
basic block, branch, and function call coverage, and "-c"
which instead of relative frequencies will use the actual
counts. For more information on the use of gcov and basic
block profiling with gcc, see the latest GNU CC manual, as
of GCC 3.0 see
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
and its section titled "8. gcov: a Test Coverage Program"
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
Pixie Profiling
Pixie is a profiling tool available on IRIX and Tru64 (aka
Digital UNIX aka DEC OSF/1) platforms. Pixie does its pro-
filing using basic-block counting.
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You can build a profiled version of perl called perl.pixie
by invoking the make target "perl.pixie" (what is required
is that Perl must be compiled using the "-g" flag, you may
need to re-Configure).
In Tru64 a file called perl.Addrs will also be silently
created, this file contains the addresses of the basic
blocks. Running the profiled version of Perl will create a
new file called "perl.Counts" which contains the counts for
the basic block for that particular program execution.
To display the results you use the prof utility. The exact
incantation depends on your operating system, "prof
perl.Counts" in IRIX, and "prof -pixie -all -L. perl" in
Tru64.
In IRIX the following prof options are available:
-h Reports the most heavily used lines in descending order
of use. Useful for finding the hotspot lines.
-l Groups lines by procedure, with procedures sorted in
descending order of use. Within a procedure, lines are
listed in source order. Useful for finding the hotspots
of procedures.
In Tru64 the following options are available:
-p[rocedures]
Procedures sorted in descending order by the number of
cycles executed in each procedure. Useful for finding
the hotspot procedures. (This is the default option.)
-h[eavy]
Lines sorted in descending order by the number of cycles
executed in each line. Useful for finding the hotspot
lines.
-i[nvocations]
The called procedures are sorted in descending order by
number of calls made to the procedures. Useful for
finding the most used procedures.
-l[ines]
Grouped by procedure, sorted by cycles executed per pro-
cedure. Useful for finding the hotspots of procedures.
-testcoverage
The compiler emitted code for these lines, but the code
was unexecuted.
-z[ero]
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Unexecuted procedures.
For further information, see your system's manual pages for
pixie and prof.
Miscellaneous tricks+ Those debugging perl with the DDD frontend over gdb may
find the following useful:
You can extend the data conversion shortcuts menu, so
for example you can display an SV's IV value with one
click, without doing any typing. To do that simply edit
~/.ddd/init file and add after:
! Display shortcuts.
Ddd*gdbDisplayShortcuts: \
/t () // Convert to Bin\n\
/d () // Convert to Dec\n\
/x () // Convert to Hex\n\
/o () // Convert to Oct(\n\
the following two lines:
((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
so now you can do ivx and pvx lookups or you can plug
there the sv_peek "conversion":
Perl_sv_peek(my_perl, (SV*)()) // sv_peek
(The my_perl is for threaded builds.) Just remember that
every line, but the last one, should end with \n\
Alternatively edit the init file interactively via: 3rd
mouse button -> New Display -> Edit Menu
Note: you can define up to 20 conversion shortcuts in
the gdb section.
+ If you see in a debugger a memory area mysteriously full
of 0xabababab, you may be seeing the effect of the
Poison() macro, see perlclib.
CONCLUSION
We've had a brief look around the Perl source, an overview
of the stages perl goes through when it's running your code,
and how to use a debugger to poke at the Perl guts. We took
a very simple problem and demonstrated how to solve it fully
- with documentation, regression tests, and finally a patch
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for submission to p5p. Finally, we talked about how to use
external tools to debug and test Perl.
I'd now suggest you read over those references again, and
then, as soon as possible, get your hands dirty. The best
way to learn is by doing, so:
+ Subscribe to perl5-porters, follow the patches and try
and understand them; don't be afraid to ask if there's a
portion you're not clear on - who knows, you may unearth
a bug in the patch...
+ Keep up to date with the bleeding edge Perl distributions
and get familiar with the changes. Try and get an idea of
what areas people are working on and the changes they're
making.
+ Do read the README associated with your operating system,
e.g. README.aix on the IBM AIX OS. Don't hesitate to sup-
ply patches to that README if you find anything missing
or changed over a new OS release.
+ Find an area of Perl that seems interesting to you, and
see if you can work out how it works. Scan through the
source, and step over it in the debugger. Play, poke,
investigate, fiddle! You'll probably get to understand
not just your chosen area but a much wider range of
perl's activity as well, and probably sooner than you'd
think.
The Road goes ever on and on, down from the door where it began.
If you can do these things, you've started on the long road
to Perl porting. Thanks for wanting to help make Perl better
- and happy hacking!

AUTHOR

This document was written by Nathan Torkington, and is main-
tained by the perl5-porters mailing list.
perl v5.8.8 2006-06-30 54